Motorola sbg6580 loses 10mbps in speed in bridge mode

I just got signed up for the Time Warner 50/5 cable Internet. When the Motorola sbg6580 is in regular Router mode, I get 50/5. When I put it in bridge mode, I get 40/5. I've tested this without a router (just my computer to the router in bridge mode) and with a Netgear router.